PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 239

for their reactor design. Tubular configurations dominate, with diameters of typically
10 cm to facilitate light irradiation. A disadvantage, in particular when scaling up such
systems, is that mass transfer may be poor, resulting in the buildup of O 2 and CO 2 ,
which reduces the productivity of algae cultivation (Ugwu et al., 2008). Another
disadvantage is the relatively high capital cost.
Also, hybrid concepts exist, where a photobioreactor acts as a kind of breeder of
algae to a prespecified concentration; in a second step, the then densely populated
algae are fed to an open system for larger-scale production.
Harvesting of microalgae, which consist of bioparticles in suspension, means
separating them from the aqueous production medium. Challenges in harvesting
follow from the relatively low algae concentration and the small sizes. Four harvesting
methods can be distinguished (Bruton et al., 2009):
1. Sedimentation
2. Filtration
3. Flotation
4. Centrifugation
It should be noted that in addition flocculation is often necessary as pretreatment of the
grown algae to improve harvesting yields. Flocculation is a process in which dispersed
particles settle out of suspension in the form of flocks or flakes induced by the addition
of some chemical. The process of flocculation is mostly used in freshwater and waste-
water. Saltwater prevents the formation of stable colloidal suspensions, thus making
flocculation ineffective.
Question : Why can stable colloidal suspensions not be formed in saltwater?
Sedimentation is a simple technique that can be applied for microalgae showing a
naturally high sedimentation rate. Apart from flocculation, also the use of ultrasonic
sound waves is a proven technique. The capital and operation costs of sedimentation
are low. A disadvantage is that it is time intensive.
Filtration is a commonlyusedmethod in industryand canbekept simpleormademore
complex as, e.g., vacuum and pressure filtration systems, which are associated with
higher costs. The biggest challenge in filtration is finding the optimal trade-off between
very small pores that cause plugging and larger pores that do not filter out all the algae.
In contrast with sedimentation, flotation results in algae floating on the water
surface from which the biomass can then be skimmed. Some microalgae strains float
intrinsically, but flotation may be assisted by bubbling air through the grown slurry or
by modification of the surface tension of the algae particles using chemicals for
improvement of the adherence to the air bubbles. Advantages are that capital and
operating costs can be kept comparatively low. A disadvantage is the low separation
efficiency especially in shallow-depth ponds.
Centrifugation is an intensified sedimentation process with a much higher effi-
ciency. The capital and operation costs are also much higher. Despite the high costs,